The present invention relates to a fluid flow guide element having an inlet opening and an outlet opening, with structuring provided on an inside wall surface of the fluid flow guide element and equipped with recesses and/or elevations, having a predetermined inclination with respect to the longitudinal axis of the fluid flow guide element in the form of a slope. In addition, this invention relates to a fluid flow apparatus equipped with such a fluid flow guide element.
A fluid flow guide element of this type constructed as a diffuser or a nozzle is already widely used in practice, especially in large-scale industrial plants for energy production and in internal combustion engines.
The diffuser serves to recover kinetic fluid energy, with an increase in the static pressure occurring in the direction of flow. In this case kinetic energy is converted to potential energy. Such an energy conversion is desirable in many technical applications.
The diffuser of a compressor converts the high kinetic energy of the flowing fluid as completely as possible into static pressure at the impeller outlet with a low loss. In the subsonic flow range, the diffuser has the form of a channel whose cross section widens in the direction of flow. The pressure gain achievable in diffuser flow depends on many parameters. It is influenced by the diffuser opening angle as well as by the flow conditions and the boundary layer coverage in the diffuser inlet.
It is known from the boundary layer theory that flows with a pressure increase are at high risk of separation. However, such separation results in high flow losses.
In particular, a sudden widening of cross section results in separation of flow due to the great pressure gradient. Therefore, in practice it is common to use diffusers having a gradual widening of the flow cross section, usually in a conical form. Opening angles of max. 10% are customary.
However, the great design length associated with very small opening angles and the resulting great frictional loss due to wall friction have proven to be disadvantageous.
The occurrence of separation in diffusers depends primarily on the pressure increase and is thus a question of diffuser geometry.
Published German patent application no. DE 12 03 232 discloses a fluid flow guide element having an inlet opening and an outlet opening, in which a structuring provided on an inside wall surface of the fluid flow guide element and formed with recesses and/or elevations has a predetermined inclination designed as a slope relative to the longitudinal axis of the flow guide element. In addition, DE 12 03 232 describes a fluid flow apparatus having a fluid flow guide element whose inside wall surface has a structuring which has recesses or elevations distributed around the inside wall surface having a slope relative to the longitudinal axis. These recesses and elevations have a constant width.
U.S. Pat. No. 3,175,571 describes a fluid flow guide element with which unwanted frictional effects on the inside wall surface of a fluid flow guide element are reduced by providing a spiral arrangement of pockets in which a low viscosity lubricant collects. The recesses and elevations have a constant width.
Additional fluid flow guide elements are also known from U.S. Pat. No. 6,213,711 (=EP 972,128), WO 03/091578 and U.S. Pat. No. 4,029,430 (=DE 26 36 524).
In addition, a diffuser design form having inside wall surfaces equipped with transverse ribs to achieve an intense flow separation is also known. This should reshape the velocity profile along the entire channel and improve the efficiency of diffuser flow due to the intense separation of flow.